This invention relates to the field of separating vapors from liquids in many two-phase mixtures such as separating steam from water in a boiler or other two-phase mixtures such as natural gas and hydrocarbons. More particularly, the invention relates to steam drum internals for separating and drying steam.
Steam generated in a subcritical pressure drum type boiler is intimately mixed with large and variable amounts of circulating boiler water. Before the steam leaves the boiler and enters the superheater, practically all of this associated boiler water must be separated from the steam. This separation must be done within a limited space in the steam drum, within a matter of seconds and under a variety of velocity, pressure and other operating condition. The pressure drop across the steam and water separators must be kept to a minimum so as not to affect the boiler circulation or water level controls. Despite many theoretical analyses of steam and water separation and a great number of hypotheses to explain these phenomena, steam and water separation in boilers retains many aspects of an art and has thus far defied complete mathematical representation.
Nearly all of the liquid and solid impurities in the steam and water mixture must be separated from the steam before it is suitable for use. Any unseparated liquid in the steam contains dissolved and suspended salts which appear as a solids impurity in the steam when the moisture is evaporated in the superheater or it is directed to a turbine or other steam driven apparatus.
The drum of a subcritical pressure boiler serves several functions, the first being that of collecting the mixture of water and steam discharged from the boiler circuits. Also, the drum houses equipment to separate the steam from the water and then purify the steam after it has been separated. The drum internals in subcritical pressure boilers not only separate water from steam but also direct the flow of water and steam to establish an optimum distribution of fluids in the boiler during all loads of boiler operation. The internals may consist of baffles which change the direction of flow of a steam and water mixture, impellers and separators which use a spinning action for removing water from steam or moisture coalescers such as screen and corrugated plate final dryers. These devices are used singly or in consort to separate and purify the steam and remove impurities from the steam leaving the boiler drum.
The space required to accommodate steam separating and purifying equipment determines drum size. Drum diameter and length should be sufficient to provide accessibility for installation and inspection and for processing the maximum flows of water and steam. Providing sufficient drum diameter and length to provide this accessibility while still maintaining a drum of a reasonable size is a significant challenge to the designer of drum internals.